A projector device is expected to illuminate images on a rectangular screen uniformly and with excellent color rendition. For this reason, extra-high pressure mercury lamps are preferred. Extra-high pressure mercury lamps include an arc tube made of quartz glass, enclosing 0.15 mg/mm3 or more of mercury and halogen therein, and a pair of electrodes facing to each other in the arc tube with a distance of 2 mm or less therebetween. The halogen is used mainly to prevent blackening of the arc tube, and inevitably causes a so-called halogen cycle in the arc tube. These discharge lamps are described in Japanese Patent Application Publication Nos. 2005-063817, 2006-079986, and 2000-231903, for example.
Unfortunately, these discharge lamps have disadvantages in that the electrodes used therein are separated from each other only by a short distance and that a large current is required for start up. This often results in deformation of the electrodes due to heat generation and blackening of the arc tube due to evaporation of the electrode materials. In view of these problems, the electrodes have been improved to have a structure that extends the lamp life.
With reference to FIG. 13, an electrode structure of such a discharge lamp will be described below. FIG. 13 is a cross sectional view of a basic structure of an extra-high pressure mercury lamp L2 for alternating current operation, as seen in the direction of a tube axis thereof. In FIG. 13, the lamp L2 includes an arc tube 80 made of quartz glass. The arc tube 80 includes an arc tube portion 81 and rod-like sealing portions 82 extending from both ends of the arc tube portion 81. In the arc tube portion 81, generally cylindrical electrodes 90 composed of tungsten are disposed face to face and each electrode 90 has an electrode rod portion 91 connected at the rear part thereof. Each electrode rod portion 91, also composed of tungsten, is embedded in the opposite sealing portion 82 for holding. Each electrode rod portion 91 is connected to a metal foil (not shown) by welding and to an external lead rod through the foil, so that the electrodes are led to the outside of the arc tube.
The electrode 90 has a head portion 92 with a projection 92A at the front end thereof, the head portion 92 being the main body of the electrode 90 and having a spherical shape. The head portion 92 has a cylindrical barrel portion 93 at the rear end thereof. The barrel portion 93 may be provided with a tungsten coil portion 94 wounded and integrally welded therearound for assisting the lamp L2 start-up. The coil portion 94 heats the front end portion of the electrode during glow discharge when the lamp is operated, and promotes the glow-to-arc transition by increasing the temperature of the end portion.